This invention relates to a method for rejuvenating or reactivating a Group VIII metal containing hydrocarbon synthesis catalyst that has undergone short term, reversible deactivation as a result of slurry phase hydrocarbon synthesis operation. More particularly, the rejuvenation method comprises treating the partially deactivated catalyst with hydrogen in the presence of hydrocarbon containing liquids, preferably the slurry phase.
During hydrocarbon synthesis the conversion of CO decreased with time. This may be due to the deactivation of the catalyst or slurry liquid properties charging limiting mass transfer. Nevertheless, the mode of deactivation of hydrocarbon synthesis catalysts is not too well understood, but is believed to be related, at least somewhat, to the mode in which the hydrocarbon synthesis is carried out; e.g., a different deactivation mode is likely present for catalyst in fixed bed operations than the deactivation mode for slurry phase operations. Thus, fixed bed processes are essentially plug flow operations involving reactant gradients as they progress through the catalyst bed whereas slurry phase operations involve sufficient backmixing tending towards a more uniform distribution of reactants and products throughout the slurry phase. For example, in a fixed bed water would not be present at the start of the reaction and would build up as the reaction progressed through the bed. However, in a slurry phase, e.g., in a slurry bubble column, because of back mixing effects, water will be present throughout the reaction slurry bed. Consequently, deactivation modes, dependent to any degree on the presence of water, will be different for fixed bed and slurry phase processes.
Hydrogen rejuvenation treatments have been employed with catalysts operated in fixed beds with, at best, limited and inconsistent recovery of hydrocarbon synthesis activity. In one case, steady state operation in the fixed bed had not been achieved, in other cases excessively high temperatures were employed, and still in other cases the hydrogen treatment was in the absence or substantial absence of hydrocarbon liquids.
Essentially complete reversal of short term, reversible catalyst deactivation of a cobalt- or ruthenium-containing catalyst, partially deactivated in slurry phase hydrocarbon synthesis operation, can be obtained by hydrogen treating, in the absence of CO, the partially deactivated catalyst in the presence of liquid hydrocarbons, preferably slurry phase hydrocarbons. The hydrogen treating which may be referred to as a catalyst rejuvenation method, allows for the recovery of a substantial portion of the initial catalyst activity, that is, start of run activity. Thus, at least about 80+% preferably at least about 90+% of initial catalyst activity is recovered, and the rejuvenation is carried out at elevated temperatures and pressures, and with sufficient liquid for fully immersing the catalyst. Generally, the rejuvenation is effected at hydrocarbon synthesis pressures, and temperatures no more than about 100xc2x0 F. (approximately 40xc2x0 C.) below reaction temperatures. The hydrogen treatment may be performed in situ or in a separate treatment vessel. Preferably, hydrogen is injected into a slurry of hydrocarbons and catalyst, preferably with sufficient energy from the hydrogen alone, to disperse the catalyst particles in the liquid. The hydrogen is also free of oxygen and can be neat or mixed with inerts such as N2, CO2, CH4, preferably nitrogen.
In a preferred embodiment, the hydrogen treatment involves conducting a slurry phase, hydrocarbons synthesis process, primarily to produce C5+hydrocarbons in which the flow of synthesis gas feed is periodically interrupted, and during those interruptions the slurried catalyst is treated with hydrogen for a period sufficient to recover all or substantially all of the initial catalyst activity. After rejuvenation, the catalyst slurry is returned to hydrocarbon synthesis operation, e.g., in a bubble column slurry reactor.
The degree of deactivation that triggers the rejuvenation can be pre-set by the operator based on a wide variety of factors known to those skilled in the art. Also, the rejuvenation can be carried out at any time and for any reason, e.g., unexpected reversible unit upsets.
Short term, reversible catalyst deactivation leading to rejuvenation preferably occurs during that period in which catalyst activity decreases to 50% of its initial, or start of run, activity, i.e., the apparent half life of the catalyst. In this discussion, catalyst activity can be considered as a measure of rate of CO conversion, and can be represented by volumetric catalyst productivity, i.e., vol CO converted/hr/vol of catalyst. Depending on the nature and severity of the process, apparent half life usually occurs in less than about 150 days on stream at hydrocarbon synthesis conditions and at design feed rates.
While catalyst activity resulting from normal operations is short term and reversible, and can be substantially completely recovered by hydrogen treatment, some types of catalyst deactivation are irreversible and not susceptible to activity recovery via the rejuvenation technique, e.g., metal agglomeration, or catalyst poisoning by virtue of graphitic carbon laydown, sulfur, sodium or other catalyst poisons.
The period of the hydrogen rejuvenation treatment should be limited to that period sufficient to maximize the recovery of catalyst activity, thereby avoiding the possibility of hydrogenolysis of the liquid with attendant carbon formation and potentially permanent catalyst deactivation.